1. Field of the Invention
The present invention relates to a multi-beam optical scanning apparatus and an image forming apparatus using the same, and particularly to a multi-beam optical scanning apparatus which is suitably usable in an image forming apparatus, such as a laser beam printer, a digital copying machine, and a multi-function printer that employ the electrophotographic process, for example, and can achieve operation with high speed and high recording density by using a light source unit having a plurality of light emitting or radiation portions.
2. Related Background Art
FIG. 23 is a cross-sectional view taken along a main-scanning direction and schematically illustrating a main portion of a conventional multi-beam optical scanning apparatus.
In FIG. 23, plural light beams emitted, for example, from a multi-beam semiconductor laser 91 with plural light emitting portions (light emitting points) are converted into approximately parallel light beams or convergent light beams by a collimator lens 92, and are incident on a cylindrical lens 94 after sizes of cross sections of these light beams are restricted by an aperture stop 93. Each light beam incident on the cylindrical lens 94 emerges therefrom without any change in a main-scanning section. With respect to a sub-scanning section, each light beam is converged by the cylindrical lens 94, and is imaged on a place close to a deflecting facet 95a of a polygon mirror 95 serving as a deflecting unit, as a linearly-focused image extending in the main-scanning direction. Each light beam is reflectively deflected and scanned by the deflecting facet 95a of the polygon mirror 95 rotating at a uniform angular speed in a direction of an arrow A in FIG. 23, and is imaged on a surface 97 to be scanned (a scanned surface) of a photosensitive drum or the like in the form of a spot by a fθ lens 96. The scanned surface 97 is scanned with the imaged spot moving at a uniform speed in a direction of an arrow B in FIG. 23. Thus, image recording is executed on the photosensitive drum surface 97 serving as a recording medium.
In such a multi-beam optical scanning apparatus, a unit for detecting a writing start position synchronous signal immediately before writing of an image signal is usually arranged to accurately control the writing start position of an image on the scanned surface.
In FIG. 23, reference numeral 78 designates a folding mirror (a BD mirror) which reflects a light beam (a BD light beam) for detection of the writing start position synchronous signal toward the side of a BD sensor 81 described later, so that a timing of a scanning start position on the photosensitive drum surface 97 can be detected. Reference numeral 79 designates a slit member (a BD slit) which is disposed at a position optically equivalent to the photosensitive drum surface 97. Reference numeral 80 designates a BD lens which serves to establish an optical conjugate relationship between the BD mirror 78 and the BD sensor 81, and compensates for a fall or inclination of the BD mirror 78. Reference numeral 81 designates an optical sensor (the BD sensor) which acts as a device for detecting the writing start position synchronous signal. Here, elements of the BD mirror 78, the BD slit 79, the BD lens 80, the BD sensor 81 and the like constitute a portion of the detecting unit (a BD optical system) for detecting the writing start position synchronous signal. In the apparatus of FIG. 23, the timing of the writing start position for image recording on the photosensitive drum surface 97 is adjusted by detecting an output signal from the BD sensor 81.
In such a multi-beam optical scanning apparatus, in the event that plural light emitting portions A and B (although two light emitting portions A and B are illustrated in FIG. 24 for the convenience of easy understanding, three or more than three light emitting portions can be similarly arranged) are arranged in a vertical direction along the sub-scanning direction as illustrated in FIG. 24, a spacing between scanning lines formed by light beams from these light emitting portions in the sub-scanning direction on the scanned surface is likely to be much larger than a spacing of a desired recording density. Accordingly, those plural light emitting portions A and B are normally arranged along a direction oblique to the main-scanning direction as illustrated in FIG. 25, and its oblique angle δ is controlled such that the spacing between the scanning lines in the sub-scanning direction on the scanned surface can be accurately adjusted in conformity with the recording density.
In the above-discussed conventional multi-beam optical scanning apparatus, since the plural light emitting portions A and B are arranged obliquely relative to the main-scanning direction, light beams emitted from the light emitting portions A and B reach different locations spaced from each other in the main-scanning direction on the deflecting facet 95a of the polygon mirror 95 as illustrated in FIG. 26, respectively, and traveling angles of the light beams reflected by the deflecting facet 95a of the polygon mirror 95 are also different from each other. Hence, light spots are imaged on different locations spaced in the main-scanning direction on the scanned surface 97, respectively (see a light beam A and a light beam B).
In such a multi-beam optical scanning apparatus, therefore, the image signal is supplied with a timing shift of a predetermined time δT such that an image location of a light beam emitted from a certain reference light emitting portion on the scanned surface can coincide with an image location of a light beam emitted from another light emitting portion.
The deflecting facet is designed at an angle indicated by 95a′ in FIG. 26 when the timing shifts by δT, and accordingly a light beam at this moment is reflected in a direction B′, i.e., reflected in the same direction (at the same angle) as that of the light beam A, leading to coincidence of the image locations of spots formed by these light beams.
In such a construction, however, the image locations of those light beams are likely to deviate from each other in the main-scanning direction in the event that a main-scanning focus variation or focus displacement (a focus displacement in an optical axial direction of the fθ lens 96) occurs for some reasons, for example, due to a positional displacement between an optical unit for holding the optical system and the scanned surface, an assemblage error at the time when an optical component is assembled in the optical unit, or the like. For example, provided that the scanned surface 97 was displaced from a regular position to a position indicated by 97′ in FIG. 26, the image locations of the light beams deviate from each other by δY1 in the main-scanning direction as can be seen from FIG. 26.
Thus, problems of a decrease in printing precision and degradation of an image quality are conventionally present due to such occurrence of the displacement δY1 in the main-scanning direction between the image locations of the light beams emitted from the light sources (the light source unit having plural light emitting portions).
As a means for solving the above-discussed problems, U.S. Pat. No. 6,489,982 (the assignee thereof is the same as this U.S. patent application) discloses technology for effectively reducing the displacement δY1 in the main-scanning direction of the image location of each light beam emitted from each of plural light sources by appropriately setting the focal length of the collimator lens, the distance between the stop and the deflecting facet of the polygon mirror, the focal length of the fθ lens in the main-scanning direction, the spacing between light emitting points of the plural light sources in the main-scanning direction, and so forth.
The construction of the above U.S. Patent is capable of lowering the displacement δY1 in the main-scanning direction of the image location of each light beam emitted from each of the plural light sources to a level that is practically allowable.
On the other hand, laser oscillation is liable to be unstable, in the event that plural light beams incident on the photosensitive drum surface are regularly reflected by the photosensitive drum surface, and are again returned to the light emitting portions such as semiconductor lasers. Further, when the regularly-reflected light returns to the optical system, there is a possibility that the reflected light is again returned to the photosensitive drum surface by reflection at a surface of the optical system, and a problem of ghost accordingly appears.
Therefore, as illustrated in FIG. 27, the construction is designed such that a principal ray of each of plural light beams incident on the photosensitive drum surface can form a predetermined angle α relative to a normal to the photosensitive drum surface in the sub-scanning direction. In such a construction, accordingly, the regularly-reflected light from the photosensitive drum surface returns to neither the semiconductor laser, nor the optical system. FIG. 27 is a cross-sectional view in the sub-scanning direction schematically illustrating a main portion of the above-discussed conventional multi-beam optical scanning apparatus using a plurality of light sources.
In the multi-beam optical scanning apparatus having such a construction, lengths of plural scanning lines formed on the photosensitive drum surface are likely to differ from each other as illustrated in FIG. 28. Hence, a displacement or variation in the main-scanning direction between the image locations of plural image spots occurs on the photosensitive drum surface, especially at its end portions in the main-scanning direction.
The displacement or variation in the main-scanning direction of the image location depends on an average α of angles formed between principal rays of the plural light beams incident on the photosensitive drum surface and the normal to the photosensitive drum surface in the sub-scanning direction, an average β of angles formed between principal rays of the plural light beams incident at any scanning location (any given scanning location) on the photosensitive drum surface and the normal to the photosensitive drum surface in the main-scanning direction, a resolution in the sub-scanning direction (a pitch of the scanning lines), and the number of simultaneously-scanned scanning lines (the number of light emitting portions of the light source unit).
In other words, the displacement or variation in the main-scanning direction of the image location on the scanned surface 97 is a sum of a positional displacement δY1 caused by the arrangement of plural light emitting portions oblique to the main-scanning direction (i.e., along the sub-scanning direction), and a positional displacement δYD caused by the arrangement in which the angle formed between the principal ray of each of plural light beams incident on the photosensitive drum surface and the normal to the photosensitive drum surface in the sub-scanning direction is set to a predetermined angle α, thereby incurring the problems of a decrease in the printing precision and degradation of the image quality.
Therefore, it can be understood from the above that it is necessary to consider not only the reduction of the positional displacement δY1 in the main-scanning direction of the image location of the light beam from each of plural light sources executed by the method disclosed in the above-identified U.S. patent, but also the positional displacement δYD caused by the arrangement in which the angle formed between the principal ray of each of plural light beams incident on the photosensitive drum surface and the normal to the photosensitive drum surface in the sub-scanning direction is set to a predetermined angle α.